Exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline, or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has, for many years, attempted to reduce the quantities of pollutants from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970's. Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically-active components for catalytic converters on automobiles.
Aluminum titanate (AT) ceramics have emerged as an excellent candidate for high-temperature applications. To achieve the desired porosity in such aluminum titanate materials, graphite pore formers have been added to the inorganic batch materials.
Hydrophobically modified cellulose polymers such as methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) have been used as binders in automotive substrate and diesel filter ceramic precursor batch compositions. These polymers give the batch the necessary plasticity and green strength in the forming and drying stages to produce high quality honeycomb ware. However, polymers such as MC and HPMC can undergo phase separation and subsequent gelation at a characteristic temperature. At such a temperature the methyl cellulosic polymers lose the water that surrounds the pendant methoxy side groups. This loss of hydration exposes the methoxy groups and enables hydrophobic associations to occur between the methoxy substituents of neighboring chains. This leads to phase separation and ultimately the build up of a long range gel network (refs. 1-5). When the binder undergoes this thermal phase transition within a ceramic precursor batch, the batch becomes stiffer and the extrusion pressure increases significantly which can produce severe defects in the extruded honeycomb structure. The thermal transition behavior of polymers like MC and HPMC can limit the extrusion process of numerous ceramic product lines. The batch temperature increases with feed rate due to increased shear heating in the extruder. Ultimately, throughput reaches a limit as the batch approaches the thermal transition temperature of the binder.